In a typical braking system of a vehicle, a brake fluid pressure (hereinafter referred to as master cylinder pressure), which is produced in a master cylinder in response to a driver's depression of a brake pedal, is transmitted to wheel cylinders for the four wheels, whereby a braking force is applied to each wheel. If a large braking force acts on each wheel in response to a deep depression of the brake pedal, while the vehicle furnished with the braking system of this type is running, the deceleration of the vehicle becomes higher to reduce the rear wheel load, so that the ground contact performance of the rear wheels is lowered. If the master cylinder fluid pressure is distributed substantially equally to the front and rear wheel cylinders with the ground contact performance of the rear wheels thus lowered, the rear wheels are locked first, so that the braking stability of the vehicle is deteriorated.
In order to avoid the deterioration of the braking stability due to the rear-wheel-first locking, as is conventionally known, proportioning control valves (PCVs) are incorporated in the braking system. For example, each of two PCVs is disposed in the middle of a duct which connects each corresponding one of fluid pressure generator sections of the master cylinder and its corresponding rear wheel. The PCVs transmit the master cylinder pressure directly to the wheel cylinders for the rear wheels if the braking force is small. If the master cylinder pressure attains a level not lower than a set pressure, on the other hand, the PCVs lower the rate of increase of the fluid pressure transmitted to the rear wheel cylinders.
Thus, in the braking system furnished with the PCVs, the rear wheel braking force increases at a high rate as the front wheel braking force increases, in a small braking force region where the input fluid pressure to the PCVs is not higher than a set pressure. In a large braking force region where the input fluid pressure to the PCVs is higher than the set pressure, on the other hand, the rear wheel braking force increases at a low rate as the front wheel braking force increases. In other words, if a curve indicative of the braking force distribution characteristics of the braking system furnished with the PCVs is drawn on a graph whose axes of ordinate and abscissa represent the rear wheel braking force and front wheel braking force, respectively, then this braking force distribution curve is composed of a first straight line with a large inclination, which corresponds to the small braking force region, and a second straight line with a small inclination, which corresponds to the large braking force region.
The braking force distribution characteristics of the conventional braking system are set so that the braking force distribution ratio for the rear wheels is lower than in the case of a braking force distribution (ideal braking force distribution) such that the four wheels are simultaneously locked when the vehicle is braked. In this manner, the braking force stability is prevented from being lowered by the rear-wheel-first locking. Thus, the conventional braking force distribution curve is situated closer to the axis of abscissa than an ideal braking force distribution curve, that is, the rear wheel braking force always takes a value smaller than that of the ideal braking force. Meanwhile, the rear wheels cannot always be locked if they are subjected to a braking force of a value greater than a value which is determined by the conventional or ideal braking force distribution curve. In other words, even if there is enough room for the increase of the rear wheel braking force, the conventional braking system produces an overall braking force by increasing the share of the front wheel braking force correspondingly.
If the share of the front wheel braking force is excessively increased in this manner, wear of braking pads of a front wheel brake unit is increased, and besides, heat release from the brake increases. Accordingly, the braked vehicle is liable to nose diving, as well as a fade, such that the friction coefficient of the brake pads is suddenly reduced, and a vapor lock which is attributable to an increase of the brake fluid temperature. Thus, the braking stability is lowered.
If the share of the rear wheel braking force is increased, however, the rear wheels becomes liable to be locked, so that their locking should be prevented.
In consideration of these circumstances, technical ideas which are designed so that the rear wheel braking force distribution can be increased without causing the rear wheels to be locked are disclosed in Published Unexamined Japanese Patent Application Nos. 1-257652 (DE3742173, FR2624462, or GB2213543), 3-125657 (GB2236156 or DE3931858), and 3-208760 (DE4029332, GB2238092, or FR2654401). These prior art examples are provided with an anti-lock device and solenoid-operated valves for normally neutralizing the action of the proportioning control valves, so that the rear wheels can be prevented from locking while enjoying an increase of the braking force distribution thereto.
In these prior art examples, however, the proportioning control valves can be actuated only in case of trouble of the anti-lock device, so that the functions of the proportioning control valves cannot be effectively utilized. Thus, the braking force cannot be properly distributed to the rear wheels.